Since the present invention will be described with reference to a television receiver having a picture-in-picture (PIP) feature, basic television theory insofar as required for a thorough understanding of the invention will be briefly summarized here.
One complete TV picture is called a frame and is composed of two fields which are offset both temporally and spatially as shown in FIG. 1. At the TV receiver the fields must be interlaced in the correct order to reconstruct the transmitted picture. This is accomplished by offsetting vertical synchronization relative to horizontal synchronization by half a horizontal line from field to field as shown in FIG. 2. The vertical synchronization pulse leading edge coincides with horizontal synchronization in field one whereas the leading edge in field two is halfway across the horizontal line.
Vertical synchronization is detected in the TV receiver by an integration process or a countdown process which destroys the exact relationship between horizontal and vertical synchronization as described above, although the half line offset is maintained. This is sufficient for TV receiver interlace, but makes field detection (i.e. determining which field is the even field and which field is the odd field when both fields are to be stored in, or read from memory) difficult.